Electronic devices are typically integrated in chips of semiconductor material. The chips are formed in large numbers in a wafer of semiconductor material through a production process typically performed in a sequence of steps. At the end of this process, the wafer includes a plurality of chips (equal to each other) that are then separated by a cutting operation.
In particular, some classes of electronic devices are formed by two chips coupled to each other.
For example, a MEMS (Micro-Electro-Mechanical System) device is formed by a sensor chip, wherein there is implemented an electro-mechanical sensor, and by a cap chip, used to cover the sensor in order to protect it from the external environment.
The production process of such class of electronic devices involves the formation of a plurality of sensor chips on a wafer, and the formation of a corresponding plurality of cap chips on another wafer. A composite wafer is obtained from the union of these two wafers by overlapping them. The MEMS devices are then obtained by cutting the composite wafer at each pair of sensor chip and cap chip.
Should the MEMS devices be found defective before or during their use, they are returned to the manufacturer in order to carry out a failure analysis; therefore, the possibility of tracing the original position of the sensor chip in the corresponding wafer is of strategic importance for the management of the quality of the production process. In fact, functional parameters, performance and reliability of each sensor chip may vary even considerably as a function of its position in the wafer. For this reason, it is important for the manufacturer to know where to intervene for improving the quality of the production process.
For such purpose, it is known in the state of the art to provide on each sensor chip an index being indicative of its position within the corresponding wafer.
In particular, with the increase in the size of the wafers and with the increase of the integration capability of electronic devices, each step of the production process of the sensor chips through a proper photolithographic mask may no more be performed in only one step (so-called “one shot” mode) over the entire area of the wafer, but it is typically performed step by step (so-called “stepper shot” mode) on different portions of the wafer; at each step, the mask acts on a corresponding area of the wafer wherein there will be made, at the end of the process, a corresponding subset of sensor chips; the same operation is repeated until applying the same mask on the entire area of the wafer. In this case, the index of each sensor chip is formed by two portions. A first index portion (obtained through a specific mask) identifies the position of the sensor chip within the corresponding area of the wafer (being equal for the sensor chips placed in corresponding positions in the different areas because of the repeated use of the same mask). A second portion of the index (obtained by known techniques of direct writing on wafer, for example, laser-based) then identifies the area of the wafer in which the sensor chip is formed.
In any case, the integration capability of the sensor chips required by the current design specifications is such that the number of sensor chips integrated in each wafer grows significantly; therefore, the number of sensor chips to be indexed being greater and greater, the index will be longer and longer. However, an increasing length of the index is inconsistent with the miniaturization of electronic devices because the length of the index may significantly affect the size of the sensor chips and, ultimately, of the corresponding electronic devices.
The problem is exacerbated by the fact that often, for facilitating the identification of the index portions, the two index portions are written in different regions of the sensor chip. Since such regions are properly spaced from the active areas of the sensor chip for avoiding damages during the writing of the index, this technique may cause further increases in the size of the electronic devices.
Other indexing techniques are known in the art.
For example, the document JP 2007 081123, which is incorporated by reference, discloses a chip being provided with a chip position identification pattern including information indicating the positions of a plurality of chip patterns in a reticle to be used in stepper lithography, and other information indicating the positions of exposure shots repeated across the semiconductor wafer. But such document does not face nor solve the above-mentioned problems relating to excessively long indexes with respect to the chip size.
The document US 2003/166313, which is incorporated by reference, discloses a semiconductor element mounting method providing for the possibility of superimposing different chips to each other at a wafer level.
The document US 2003/127718, which is incorporated by reference, discloses a semiconductor component and a method for identifying a semiconductor component that comprises at least one semiconductor substrate equipped with electronic/electro-mechanical components, which semiconductor substrate—except for its leads—is embedded in a housing part made of plastic.
The document WO 01/39269, which is incorporated by reference, discloses a system for making small modifications to the pattern in standard processed semiconductor devices, such modifications being made to create a small variable part of the pattern against a large constant part of the same pattern.